Transmission phantom for total performance assessment of scintillation camera imaging and method of manufacture

ABSTRACT

A transmission phantom for total performance assessment of scintillation camera imaging has a constant thickness and at least two layers of homogenous material each layer having different radiation attenuating properties. A layer having a moderate attenuation provides relief areas and a radiation absorbing layer conforming to the relief areas provides for variable transmission through the phantom corresponding to the thickness of the absorbing layer to simulate pathological conditions in a patient. The method of manufacture includes the preparation of a relief mold including a bottom plate and side walls. The radiation absorbing material is poured into the mold, allowed to hardened and milled to a flat surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns the necessary physical and chemical compositionand construction of a transmission phantom for total performanceassessments in scintillation camera imaging. In particular, thetransmission phantom of this invention concerns the simulation ofcomplex distribution patterns of radionuclides, for example, as obtainedin bone scintigraphy. When placed on top of a uniform source ofradioactivity, the phantom creates a photon intensity distributiondirected towards the detector that is equivalent to that which isobserved in a patient study. The design technique also enablessimulations of studies where there are complex distributions ofradioactivity within the human body in front of the camera, e.g. in bonescintigraphy. The invention further concerns a method for massproduction of this phantom.

2. Description of the Related Art

A scintillation camera (gamma camera) is currently used in nuclearmedicine to acquire images of photon-emitting radionuclides distributedwithin the patients body following an intravenous injection, afterbreathing a radioactive gas or after an oral intake of radionuclides.Such cameras are used in most larger hospitals. In Norway they are usedin 23 hospital laboratories. Artificially produced radionuclides areused, and they are required to be photon-emitters and have relativeshort half lives.

These cameras perform an indirect imaging, in the sense that thedetection process is based on electronic circuits that must be correctlyadjusted if artificial structures (artefacts), not related to thedistribution of radionuclides, shall be avoided. Most laboratoriestherefore posess simple constructions made of lead to be used togetherwith radioactive sources for monitoring camera quality.

Devices for quality control of scintillation cameras are previouslydescribed. U.S. Pat. No. 4,408,124 comprises a lead plate, impervious togamma rays, with an orthogonal array of apertures sandwiched between tworectangular sheets of lucite, while U.S. Pat. No. 4,419,577 comprises aradiation transparent, closed, planar body member with internalmercury-filled communication passages that define a calibrated radiationopaque test pattern. These devices are suitable for testingscintillation camera performance (uniformity, linearity, intrinsicresolution). They do, however, not simulate the complex distribution ofradioactivity in a patient since, 1) the gamma impervious lead plate inU.S. Pat. No. 4,408,124 is of constant thickness, and the apertures areof uniform size; and 2) the low attenuating body member in U.S. Pat. No.4,419,577 is planar, of constant thickness, and the mercury bars allhave uniform thickness and attenuation, and thus represent only twolevels of transmission (mercury bar/lucite, lead plate/apertures).

From the literature, hollow containers, filled with radioactivesolutions are known, and in which objects of different materials may beinserted to displace the radioactive solution and thereby simulateuptake defects. Also, simple transmission phantoms have been built ascombinations of several layers of absorptive material (e.g. copper). Bythe use of a uniform source behind such a phantom, a transmitted photonintensity distribution resembles that obtained in a patient study. Noneof these constructions re suited for simulations of more complexdistributions of radioactivity in the patient, such as when theradionuclides are accumulated in bone tissues.

SUMMARY OF THE INVENTION

The object of the present invention is thus to construct transmissionphantoms which, together with radioactive sources, simulate the complexdistribution of radioactivity in patients. This is obtained by thetransmission phantom of the present investigation as it is characterizedby the patent claims.

In the present invention the transmission varies continuously over thephantom surface in such a manner that the resulting image contains awide variety of image intensities. Thus the transmission phantom of thepresent invention allows the laboratory to test their own diagnosticprocedures.

By means of the present method, the transmission phantom can be designedin such a way that it most exactly simulates the photon intensitydistribution obtained in static studies, i.e. studies, in which there isno time variation of the radioactivity distribution during theexamination time. We have already registered a certain demand for such aphantom in the market and concluded that a mass production is feasiblewith relatively simple means.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional through an elevational view of thetransmission phantom of this invention showing relief areas of varyingthickness formed from a layer of material having low radiationattenuating properties and a covering layer of radiation absorbingmaterial conforming to hte relief areas.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The method of production consists of construction, after carefulmeasurements of image intensities in the image to be simulated andcalculations of absorbancies, of a relief a, from a material with lowradiation attenuation properties (different types of plastics, epoxyresins, polyacrylate etc). The relief is modelled in clay.

This relief 1 is covered to a certain thickness with a mixture 2,characterized by a considerably greater attenuation. This compositionconsists of barium sulphate powder mixed with a two-component epoxyresin (Ciba-Geigy) of specific viscosity. The epoxy is prepared beforethe barium sulphate powder is added, and the composition subjected tovacuum for a short time in order to remove air bubbles. Followinghardening of the epoxy the phantom is processed by a milling cutter inorder to produce a flat surface. Barium in a different chemical compoundor other elements characterized by sufficiently high atomic number inappropriate compounds may also be used. Other compounds can substitutethe epoxy as a binder. Thus the transmission phantom, due to thepre-prepared relief 1 with a thickness pattern reflecting a radioactiveemission pattern from a patient in such a way that when the relief 1 isthick the adjacent layer of attenuating material 2 will be thin, with areduced attenuation and consequently low absorption of photons emittedfrom the source behind the phantom.

The varying thickness of the absorbing material 2 produces varyingintensity of photons transmitted towards the detector of thescintillation camera. The resulting image very closely resembles thatobtained in a patient study. Illnesses producing differentconcentrations of radioactivity or uptake defects (less local activity)can be simulate by appropriate changes of the relief 1 height.

Mass production is effectuated by making a casting of the clay relief,either by using epoxy together with an appropriate slip material, orconstructing special tools for pressing plastics. The relief may bebuilt up in epoxy resin or different types of plastics, characterized bysatisfactory absorption qualities.

The final product is formed like a block, measuring in the presentversion 40×40 cm (bigger phantom is possible) with a maximum thicknessof 5 cm. It may be applied in combination with a uniform source ofradioactivity for education of personel, optimalization of imaging anddisplay techniques, verification of computer image processing and forconstancy testing of the equipment. The phantom may contain sets ofplugs having different attenuation properties which can be used to varythe contrast between the normal structures and the simulatedpathological areas. The standard version of the phantom comprises alarger number of simulated accumulations and uptake defects.

I claim:
 1. A transmission phantom for total performance assessment ofscintillation camera imaging by measurement of photon transmissionthrough the phantom, said phantom having a substantially uniformthickness comprising a first layer of low radiation attenuatingmaterial, said first layer being formed with selected contours definingrelief areas, a second layer of material conforming to the relief areasof the first layer, said second layer being adapted to absorb radiationand to provide variable distribution intensity of photon transmission incorrespondence with the shape of the relief areas to thereby simulatepathological images.
 2. A transmission phantom as claimed in claim 1wherein the second layer is comprised of a homogenous mixture includinga barium compound and a binder.
 3. A transmission phantom as claimed inclaim 2 wherein the barium compound is barium sulphate powder.
 4. Atransmission phantom as claimed in claim 2 wherein the binder is anepoxy resin.
 5. A method for manufacturing a transmission phantom fortotal performance assessment of scintillation camera imaging comprisingthe steps of:determining relief areas for effecting a selecteddistribution of photon transmission through the phantom corresponding toimage intensities in a pathological image to be simulated, forming afirs layer of a low radiation attenuating material defining the reliefareas, and covering the first layer with a radiation absorbing materialto form a second layer.
 6. A method for manufacturing a transmissionphantom as claimed in claim 5 wherein the first layer is formed of aplastic material.
 7. A method for manufacturing a transmission phantomas claimed in claim 5 wherein the second layer is formed of a settableviscous composition containing barium.
 8. A method for manufacturing atransmission phantom as claimed in claim 7 wherein the composition isvacuum treated to remove entrained air bubbles before setting.
 9. Amethod for manufacturing a transmission phantom as claimed in claim 7including the further step of mechanically treating the compositionafter setting to produce a flat surface.
 10. A method for manufacturinga transmission phantom as claimed in claim 5 further including the stepof preparing a mold defining the relief areas and pouring the radiationabsorbing material into the mold.
 11. A method for manufacturing atransmission phantom as claimed in claim 10 wherein the relief areas aremolded from clay.